I have had a mild version of this since I returned from Vietnam, and I started taking increased Vitamin D because of my age. I too got significantly reduced hives, and now I guess I know the reason....
Over 12 weeks, 38 study participants daily took a triple-drug combination of allergy medications (one prescription and two over-the-counter drugs) and one vitamin D3, an over-the-counter supplement. Half of the patient's took 600 IUs of vitamin D3 and the other half took 4000 IUs.
Researchers found after just one week, the severity of patients' symptoms decreased by 33 percent for both groups. But at the end of three months, the group taking 4000 IUs of vitamin D3 had a further 40 percent decrease in severity of their hives. The low vitamin D3 treatment group had no further improvement after the first week.